Fundamentally, graphene consists of a single layer of graphite (i.e., sp2 hybridized carbon atoms). Graphene is approximately two hundred times stronger than steel, nearly one million times thinner than a human hair, and more conductive than copper. With such unique and beneficial physical properties, graphene, and in particular, high quality graphene, is desirable for use in various industries. For example, obtaining high quality graphene is of significant importance for electronic and photonic based applications. Currently, chemical vapor deposition method is the preferred route of manufacturing high quality graphene for these applications. Chemical vapor deposition, however, is expensive and cannot currently produce the quantities of graphene demanded for large-scale industrial applications at a reasonable cost.
Because of its unique and beneficial properties, significant research and development work has recently been undertaken to cost-effectively produce high quality graphene on a commercial scale. One such method considered that is capable of producing large quantities of graphene is chemical reduction of graphene oxide. The graphene created through reduction of graphene oxide has traditionally been of an inferior quality as compared with graphene produced through chemical vapor deposition due to defects (discussed below) that are created during the manufacturing process. Graphene produced through reduction of graphene oxide is currently used in developing new technologically advanced materials specifically in the areas of nanocomposites, functional coatings, paints and electrode materials for chemical and biological sensing and energy storage devices.
One of the most common methods of creating graphene oxide is through the chemical exfoliation of graphite (e.g., bulk graphite), which consists of a large number of graphene sheets held together by Van der Waals forces. One source of excellent, high quality pure bulk graphite is Sri Lankan vein graphite. Sri Lanka has a longstanding reputation for its high quality crystalline vein graphite with purity levels ranging from 80-99% carbon. Sri Lankan vein graphite is mined as lumps and is considered to have a high degree of crystalline perfection, excellent electrical and thermal conductivities, and superior cohesive energy as compared to other natural graphite materials.
In a traditional chemical exfoliation method, graphite is treated with a strong oxidizing agent to produce graphene oxide. One of the earliest recorded methods of synthesis of graphene oxide was by Brodie (1859). Brodie demonstrated the synthesis of graphene oxide by adding a portion of potassium chlorate to a slurry of graphite in fuming nitric acid. Subsequent studies by Staudenmaier (1898) improved upon Brodie's method by using concentrated sulfuric acid as well as fuming nitric acid and adding the potassium chlorate in multiple aliquots over the course of the reaction. Staudenmaier's alteration of Brodie's method helped the production of a highly oxidized graphene oxide in a single reaction vessel significantly more practical. Hummers (1958) further improved upon this method (see Hummers et al, 1958, herein “Hummer”). In Hummers's method, which is commonly used today, graphite is oxidized by treatment with KMnO4 and NaNO3 in concentrated H2SO4.
These traditional methods of producing graphene oxide are not devoid of flaws. While each of Brodie's, Staudenmaier's, and Hummers's methods can be used to create graphene oxide, each results in a graphene oxide structure that is less than ideal for the creation of high quality graphene through reduction on a commercial scale. More specifically, each of these methods results in significant defects in the graphene oxide chemical structure, defects which are not readily repairable during a subsequent reduction of graphene oxide to graphene. For example, defects can form in Hummers's method because oxidation of graphite with KMnO4 results in the formation of manganate ester which will create a vicinal diol. If left unprotected, the vicinal diol may be oxidized to diketone, which leads to the formation of holes in the graphene basal plane. Such chemical defects in the resulting chemically converted graphene diminish the highly sought after electrical and mechanical properties as compared with pristine, high quality graphene. Further, each of these prior art methods involves the generation of one or more toxic gases, such as NO2, N2O4, and/or ClO2.
Recently an improved version of Hummers's method was disclosed by James Tour's group at Rice University (see US 2012/0129736 A1, herein “Tour”). This improved method excludes NaNO3, requires a higher amount of KMnO4 and H2SO4, and also performs the reaction in a 9:1 mixture of H2SO4/H3PO4. According to Tour, this method does not generate toxic gasses and prevents excessive oxidation and defect (i.e., hole) formation in the resulting graphene oxide. Also according to Tour, it is the addition of H3PO4 that helps to prevent defect formation, which can be caused by excessive oxidation in the graphene oxide structure. More recently Chen and co-workers (see Chen et al, “2013”, herein “Chen”) introduce a method without using H3PO4, but the oxidation method gives lower oxidation than the Tour's.
The use of H3PO4, however, is undesirable due to its cost and the increased complexity of the reaction method. Moreover, KMnO4 is one of the strongest oxidants, especially in acidic media. Complete intercalation of graphite with concentrated H2SO4 can be achieved with the assistance of KMnO4 by forming graphite bisulfate (see Sorokina et al, 2005). Accordingly, the formation of graphite bisulfate gives reaction stability, so the role of NaNO3 and/or H3PO4 is unnecessary for the synthesis of graphene oxide (herein “GO”) using Hummers method. Accordingly, it would be beneficial to create a commercially viable method of creating high quality, highly oxidized graphene oxide (i.e., graphene oxide with fewer defects) from bulk graphite without the creation of toxic gasses or other toxic byproducts or the use of H3PO4.